1. Field of the Invention
The present invention relates to an optical switch, and in particular, to a mechanical optical switching device having multiple input optical fibers and multiple output optical fibers.
2. Description of the Related Art
As for an optical switch for switching an optical path, those for switching a traveling direction of light by electrically changing a refractive index or phase of an optical path, switching a traveling direction of light by mechanically displacing an optical path, and so on have been developed. The mechanical optical switch has been often used in an optical communication apparatus, optical transmission apparatus, or the like because it has a low coupling loss of light, is substantially independent of the wavelength of the propagating light, and have a self-latching property for maintaining, even after removal of electric power, the coupling state of light in a state before the removal.
The mechanical optical switch comprises a movable optical fiber which can be elastically deformed and two fixed optical fibers, an open end of the movable optical fiber facing to open ends of the fixed optical fibers via an optical gap, and switches the optical path by displacing the open end of the movable optical fiber with respect to the open ends of the fixed optical fibers. In the mechanical optical switch, the movable optical fibers and the fixed optical fibers are usually used as input paths and output paths, respectively. A silicone based liquid or the like serving as a refractive index matching oil is placed between the open end of the movable optical fiber and those of the fixed optical fibers in order to prevent attenuation and scattering of light from occurring there. For that purpose, the whole mechanism of the optical switch is contained in an air-tight housing, and the housing is filled with the silicone based liquid or the like.
The fixed optical fibers are held by a fixed holder (fixed block) at portions close to their open ends. The movable optical fiber is held by a movable holder (movable block) at a portion close to its open end. The movable optical fiber is held by another fixed holder at a point distant from the tip of the movable optical fiber, and the point constitutes a fulcrum.
Since the open ends of the fixed optical fibers and the open end of the movable optical fiber are provided to face to each other, the fixed holder and the movable holder also face to each other. In order to displace the open end of the movable optical fiber with respect to the open ends of the fixed optical fibers, the movable holder is displaced with respect to the fixed holder. In order to keep such a movement within a certain route for avoiding misalignment when displacing the movable holder with respect to the fixed holder, guide pins are provided on one of the holders for example, fixed holder) to protrude from the facing surface thereof and are inserted into guide channels provided on the facing surface of the other holder (for example, movable holder). Thus, when the movable holder is displaced, the guide pins move along the guide channels, and the movable holder is stopped when the guide pins reach the ends of the guide channels.
An electromagnetic actuator is used to displace the movable holder or movable optical fiber with respect to the end of the fixed optical fiber, which is typically large in size. The movable holder or movable optical fiber is moved in the refractive index matching oil having viscosity, so that a significant magnitude of force is required. For that purpose, a large electromagnetic actuator is required. The housing of the optical switch is intended to contain the large electromagnetic actuator therein, so that it also becomes large in size. In addition, since the optical switch circuit includes a combination of many optical switches, if the individual optical switches are large, the optical switch circuit is also large.
U.S. Pat. No. 6,169,826 (issued on Jan. 2, 2001) has been proposed to reduce the size of an electromagnetic actuator used in an optical switch. The structure thereof will be described below with reference to FIGS. 10 and 11.
Referring to FIGS. 10 and 11, in an optical switch 800, fixed optical fibers 824 and movable optical fibers 822 are positioned so as to have their respective open ends faced to each other in a housing 810. The open ends of the movable optical fibers are moved relatively to the open ends of the fixed optical fibers to connect and/or disconnect optical paths. The fixed optical fibers are held by a fixed holder 832 made of soft magnetic ceramic at a portion close to the open ends thereof in the housing 810. The movable optical fibers 822 are supported and fixed in the housing 810 by another fixed holder 836 at a distance from the open ends thereof and are held by a movable holder 834 made of soft magnetic ceramic at a portion close to the open ends thereof. When the movable holder 834 made of soft magnetic ceramic is reciprocated with respect to the fixed holder 832 made of soft magnetic ceramic, the open ends of the movable optical fibers held by the movable holder 834 made of soft magnetic ceramic are reciprocated with respect to the tips of the fixed optical fibers 824 along with the movable holder 834 to connect and/or disconnect the optical paths.
An electromagnetic actuator 850 comprises an E-shaped yoke 852 having a back yoke (column yoke) 854 which is located on the side of the fixed optical fibers 824 from the fixed holder 832 made of soft magnetic ceramic in the housing, and first and second end legs 856 and 856′ of the E-shaped yoke 852 extend from the back yoke 854 to the side surfaces of the movable holder 834 made of soft magnetic ceramic. The first and second end legs 856 and 856′ have first and second pole pieces 858 and 858′, respectively, which face the side surfaces of the movable holder 634 made of soft magnetic ceramic. The movable holder 834 made of soft magnetic ceramic can reciprocate between the first and second pole pieces 858 and 858′. A center leg 862 protruding from the center of the back yoke 854 toward the movable holder 834 made of soft magnetic ceramic is constituted by a permanent magnet 864 and the fixed holder 832 made of soft magnetic ceramic. For example, the permanent magnet 864 may be a sintered neodymium-iron-boron permanent magnet.
The permanent magnet 864 is magnetized in a direction from the fixed holder 832 made of soft magnetic ceramic to the back yoke 854 or in the direction opposite thereto. Part of the magnetic flux exiting from the permanent magnet 864 enters the first end leg 856 through the back yoke 854. Then, it enters the movable holder 834 made of soft magnetic ceramic via the first pole piece 858. Then, it passes through the fixed holder 832 made of soft magnetic ceramic to return to the permanent magnet 864. In this way, the permanent magnet 864, a first half of the back yoke 854, the first end leg 856, the first pole piece 858, the movable holder 834 and the fixed holder 832 constitute a first magnetic path. The magnetic flux of the permanent magnet passing through the first magnetic path is denoted by reference symbol A in this drawing.
Part of the magnetic flux exiting from the permanent magnet 864 enters the second end leg 856′ through the back yoke 854. Then, it enters the movable holder 834 made of soft magnetic ceramic via the second pole piece 858′. Then, it passes through the fixed holder 832 made of soft magnetic ceramic to return to the permanent magnet 864. In this way, the permanent magnet 864, a second half of the back yoke 854, the second end leg 856′, the second pole piece 868′, the movable holder 834 and the fixed holder 832 constitute a second magnetic path. The magnetic flux of the permanent magnet passing through the second magnetic path is denoted by reference symbol B in this drawing.
FIG. 10 shows a state in which the movable holder 834 made of soft magnetic ceramic is attracted by the first pole piece 858, and there is a wider gap between the movable holder and the second pole piece 858′. The optical switch 800 comprises four fixed optical fibers 824 (denoted by reference symbols f1, f2, f3, and f4 from the left) and two movable optical fibers 822 (denoted by reference symbols m1 and n2 from the left). When the movable holder 834 made of soft magnetic ceramic is attracted by the first pole piece 858, the fixed optical fiber f1 and the movable optical fiber m1 have their open ends face each other, and the fixed optical fiber f3 and the movable optical fiber m2 have their open ends face each other, thereby establishing optical paths between them respectively. On the other hand, when the movable holder 834 made of soft magnetic ceramic is attracted by the second pole piece 858′, optical paths are established between the fixed optical fiber f2 and the movable optical fiber m1, and between the fixed optical fiber f4 and the movable optical fiber m2. Displacing the movable holder 834 from the first pole pieces 858 to the second pole piece 858′ can switch the position of the movable optical fiber m1 from the fixed optical fiber f1 to f2, and the position of the movable optical fiber m2 from the fixed optical fiber f3 to f4.
First and second coil members 872 and 872′ are wound around the first and second end legs 856 and 856′, respectively. When a current for canceling or decreasing the magnetic flux A is applied to the first coil member 872, and a current having a direction intended to increase the magnetic flux B is applied to the second coil member 872′, the attraction between the movable holder 834 and the first pole piece 858 is vanished, and then the movable holder 834 is attracted to the second pole piece 858′ to move toward the second pole piece 858′. When the movable holder 834 is attracted by the second pole piece 858′, the optical paths are established in such a manner that the movable optical fiber m1 is connected to the fixed optical fiber f2 and the movable optical fiber m2 is connected to the fixed optical fiber f4. If the current applied to the first and second coil members 872 and 872′ is stopped in this state, the state in which the movable holder 834 is attracted by the second pole piece 858′ is maintained by the permanent magnet 864.
If a current for canceling or decreasing the magnetic flux B is applied to the second coil member 872′, and a current for increasing the magnetic flux A is applied to the first coil member 872 when the movable holder 834 is attracted by the second pole piece 858′, the movable holder 834 leaves the second pole piece 858′ and moves toward the first pole piece 868. Then, as shown in FIG. 10, the movable optical fiber m1 is connected to the fixed optical fiber f1, and the movable optical fiber m2 is connected to the fixed optical fiber f3. If the current applied to the coil members 872 and 872′ is stopped in this state, the connections are maintained.
In case of the electromagnetic actuator according to the above-described US patent, downsizing is realized by utilizing the movable holder 834 and fixed holder 832 both made of soft magnetic ceramic as part of the magnetic circuit of the electromagnetic actuator 850.
Since the fixed holder 832 is utilized as part of the magnetic circuit, however, most parts of the electromagnetic actuator 850 are provided on the side of the fixed optical fiber 824 in the optical switch 800. The length of the movable optical fiber 822 from its fulcrum to its open end is required to be enough for allowing a portion thereof from the fulcrum to the open end, in particular, to the point supported by the movable holder, to elastically pivot without undergoing an excessive force. For this reason, this portion of the movable optical fiber cannot be shortened.
Therefore, the optical switch has a length more than the sum of the length from the fulcrum to the open end of the movable optical fiber and the length of the electromagnetic actuator. The electromagnetic actuator includes the coil members. If the length of the electromagnetic actuator is reduced, the length of the coil member should also be reduced. Therefore, in order to ensure the same level of ampere-turn, the coil is rolled up to increase the number of overlap accordingly. As a result, the coil has an increased diameter.
The optical switch is provided on the bottom of a lower half 811 of the housing made of alumina ceramics or the like and is covered by the upper half 812 of the housing from the above, and a gap between the lower half and the upper half is sealed to provide the air-tight housing 810 as shown in FIG. 11 in a perspective view. Refractive index matching oil is filled into the housing 810 through its inlet 817 which is closed thereafter so that the mechanical optical switching structures are completely immersed in the refractive index matching oil.
The assignee of this invention proposed in U.S. patent application Ser. No. 09/993,649 (filed Nov. 27, 2001) an optical switch further smaller than that proposed in the US patent. The optical switch 900 will be described below, referring to FIG. 12.
Fixed optical fibers 924 near their open ends are fixed by a fixed holder 932 attached to a base plate 915 near a housing side wall. Movable optical fibers 922 are fixed, at a distance from the open ends thereof, to the base plate 915 by another fixed holder 936 attached to the base plate 915. The latter fixed holder 936 is arranged near another housing side wall having a slot 913 passing the movable optical fibers 922 therethrough. A movable holder 934 made of soft magnetic material is provided on the base plate 915 so as to face the fixed holder 932, holds the movable optical fibers 922 near the open ends thereof, and reciprocates relatively to the fixed holder 932 on the base plate 915, thereby connecting and/or disconnecting, or switching, the open ends of the fixed optical fibers and the open ends of the movable optical fibers. Providing a gap between the movable holder 934 of soft magnetic material and the base plate 915, such as a glass plate, allows the movable holder 934 to move smoothly.
In order to reciprocate the movable holder 934 made of soft magnetic material relatively to the fixed holder 932, an electromagnetic actuator 950 is located in an area on the side of the movable optical fibers 922 from the open ends of the movable optical fibers in the housing 910. That is, the electromagnetic actuator 950 is provided between the front end of the movable holder 934 near the fixed holder 932 and the rear end of the fixed holder 936 (that is, the end of the fixed holder 936 near the housing side wall on the side of the movable optical fiber).
As shown in FIG. 12, the electromagnetic actuator 950 has an E-shaped yoke 952, which has two end legs 956, 956′ and a center leg 962. The E-shaped yoke 952 has a back yoke 954, and the end leg 956 extends from one end of the back yoke 954 to a position where it faces one side surface of the movable holder 934 of soft magnetic material. The end leg 956′ extends from the other end of the back yoke 954 to a position where it faces the other side surface of the movable holder 934 of soft magnetic material. The end legs 956 and 956′ have pole pieces 958 and 958′, respectively, each of which faces a side surface of the movable holder 934.
The end leg 956 and the half of the back yoke 954 on the side of the end leg 956 may be collectively referred to as a first yoke. The end leg 956′ and the half of the back yoke 954 on the side of the end leg 956′ may be collectively referred to as a second yoke. The pole piece 958 attached to the end leg 956 may be referred to as a first pole piece, the pole piece 958′ attached to the end leg 956′ may be referred to as a second pole piece.
The movable holder 934 has gaps between the first pole piece 958 and one side surface of the movable holder 934 and between the second pole piece 958′ and the other side surface of the movable holder 934, respectively, so as to reciprocate between the first and second pole pieces 958 and 968′.
A permanent magnet 964 and a soft magnetic material block 966 are provided on the center leg 962 attached to the back yoke 954 of the E-shaped yoke 952, and the center leg 962 extends toward the movable holder 934. The permanent magnet 964 is magnetized in a direction from the soft magnetic material block 966 to the back yoke 954, or in the direction opposite thereto. The permanent magnet 964 may be a sintered neodymium-iron-boron permanent magnet. Part of the magnetic flux exiting from the permanent magnet 964 enters the end leg 956 through the back yoke 954. Then, it enters the movable holder 934 made of soft magnetic material via the first pole piece 958. Then, it passes through the soft magnetic material block 966 to return to the permanent magnet 964. In this way, the permanent magnet 964, the first half of the back yoke 954, the end leg 956, the first pole piece 958, the movable holder 934 and the soft magnetic material block 966 constitute a first magnetic path. The magnetic flux of the permanent magnet passing through the first magnetic path is denoted by reference symbol A in the drawing.
Part of the magnetic flux exiting from the permanent magnet 964 enters the end leg 956′ through the back yoke 954. Then, it enters the movable holder 934 made of soft magnetic material via the second pole piece 958′. Then, it passes through the soft magnetic material block 966 to return to the permanent magnet 964. In this way, the permanent magnet 964, the second half of the back yoke 954, the end leg 956′, the second pole piece 958′, the movable holder 934 and the soft magnetic material block 966 constitute a second magnetic path. The magnetic flux passing through the second magnetic path is denoted by reference symbol B in the drawing.
A first coil member 972 and a second coil member 972′ are wound around the end legs 956 and 956′, respectively. The first and second coil members 972 and 972′ are connected in series in such a manner that when a DC voltage is applied between their respective terminals 976 and 976′, the two coil members 972 and 972′ generate magnetic fields of directions opposite to each other. When a current for canceling or decreasing the magnetic flux A is applied to the first coil member 972, and a current having a direction intended to increase the magnetic flux B is applied to the second coil member 972′, the attraction between the movable holder 934 and the first pole piece 958 is vanished, and then the movable holder 934 is attracted to the second pole piece 958′ to move toward the second pole piece 958′. In the state in which the movable holder 934 is attracted by the second pole piece 958′, the optical paths are established in such a manner that the movable optical fiber m1 is connected to the fixed optical fiber f2 and the movable optical fiber m2 is connected to the fixed optical fiber f4. If the current applied to the first and second coil members 972 and 972′ is stopped in this state, the state in which the movable holder 934 is attracted by the second pole piece 958′ is maintained by the permanent magnet 964.
If a current for canceling or decreasing the magnetic flux B is applied to the second coil member 972′, and a current for increasing the magnetic flux A is applied to the first coil member 972 when the movable holder 934 is attracted by the second pole piece 958′, the movable holder 934 leaves the second pole piece 958′ and moves toward the first pole piece 958. Then, the movable optical fiber m1 is connected to the fixed optical fiber f, and the movable optical fiber m2 is connected to the fixed optical fiber f3. If the current applied to the coil members is stopped in this state, the connections are maintained.
The optical switches described in U.S. Pat. No. 6,169,826 and U.S. patent application Ser. No. 09/993,649 have electromagnetic actuators provided in an air-tight housing, which actuators move movable optical fibers at the input side to switch the optical paths. The movable optical fibers are immersed in a refractive index matching oil and, when moving the movable optical fibers, they receive viscous resistance of the refractive index matching oil. Since a large driving force is required for the reason that elastic resistance of the optical fiber becomes large when the number of movable optical fibers increases, a large-sized electromagnetic actuator is also required when the number of movable optical fibers increases. Hence, with respect to the optical switch, if expressed by a ratio of the number of input optical fibers and the number of output optical fibers, a ratio of the number of inputs and the number of outputs is 1:2 or 2:4, that is, those of a 1×2 type or a 2×4 type are used in large quantities.
However, in order to switch the optical paths, the optical switch having the number of optical fibers which match the number of paths presently used is required and a 4×8 type or a larger type is also required. There are often cases where, in order to construct any m×n optical switches, the optical switches having a small number of input optical fibers and a small number of output optical fibers such as the 1×2 type, the 2×4 type are combined plurally in use. Such an example is disclosed in Japanese Patent Laid-Open No. 6-208065 (JP 6-208065 A). Illustrated there is a 1×8 type optical switch, which was fabricated by combining seven 1×2 type optical switches in three stages.
When the 1×8 type optical switch is fabricated according to JP 6-208065 A by using a plurality of the 1×2 type optical switches disclosed in the above described U.S. Pat. No. 6,169,826 and U.S. patent application Ser. No. 09/993,649, it forms a combination shown in FIG. 13 in a plane view. In the same drawing, reference numeral 800 denotes the 1×2 type optical switch, and an input signal enters the 1×2 type optical switch located at the first stage from the input optical fiber. The output of the ×2 type optical switch of this first stage is connected to the input optical fibers of the two 1×2 type optical switches located at the second stage. Each of outputs of the 1×2 type optical switches of the second stage is connected to the input optical fibers of the four 1×2 type optical switches located at the third stage. Since the output optical fibers of the 1×2 type optical switches of the third stage are eight in total, it is obvious that the 1×8 type optical switch is constructed by the seven 1×2 type optical switches.
Because of the requirement to downsize the optical switch circuits, efforts are made to downsize individual 1×2 optical switches. Nevertheless, the 1×8 type optical switch fabricated in this way by combining the seven 1×2 type optical switches are large in size.